Acoustic wave sensors are so named because their detection mechanism is a mechanical, or acoustic, wave. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. Changes in velocity can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity being measured.
Virtually all acoustic wave devices and sensors use a piezoelectric material to generate the acoustic wave. Piezoelectricity refers to the production of electrical charges by the imposition of mechanical stress. The phenomenon is reciprocal. Applying an appropriate electrical field to a piezoelectric material creates a mechanical stress. Piezoelectric acoustic wave sensors apply an oscillating electric field to create a mechanical wave, which propagates through the substrate and is then converted back to an electric field for measurement. Depending on the type of acoustic wave sensor, different metal electrode configurations are used to establish an electric field driving the piezoelectric material. For example, when the acoustic wave is a transverse bulk wave, such as in a thickness shear mode (TSM) sensor, the electrodes are planar and they sandwich the piezoelectric material. In surface launched acoustic wave sensors the electrodes are typically interdigitated (IDT) electrode pairs, fabricated by photolithography directly onto the piezoelectric substrate.
Acoustic wave devices are described by the mode of wave propagation through or on a piezoelectric substrate. A wave propagating through the substrate is called a bulk wave. The most commonly used bulk acoustic wave device is the thickness shear mode (TSM) resonator.
When the acoustic wave propagates on the surface of the substrate, it is known as a surface wave. The surface acoustic wave sensor (SAW) and the shear-horizontal surface acoustic wave (SH-SAW) sensor are the most widely used surface wave devices. One of the important features of a SH-SAW sensor is that it allows for sensing in liquids. This is because, since the shear horizontal wave is confined to the surface of the sensor, it does not dissipate energy into liquids contacting that surface, allowing liquid operation without damping.
Of all the known acoustic sensors for liquid sensing, the Love wave sensor, a special class of the SH-SAW, has the highest sensitivity. To make a Love wave sensor, a waveguide coating is placed on a SH-SAW device such that the energy of the shear horizontal waves is confined and focused in that coating. To form a complete conventional sensor, a biorecognition coating (e.g., one including capture agents) is then placed on the waveguide coating. An immobilization chemistry layer is interposed between the biorecognition and waveguide coatings, to act as a tie layer between the two. Binding of a bio-analyte to the biorecognition coating will change the propagation characteristics of the surface acoustic wave and measuring these changes can be used to quantitatively detect the existence of the analyte.
Waveguide materials are important in the propagation of acoustic energy, particularly with respect to the construction of delay-line devices. Just as with optical waveguides, acoustic energy is propagated in the direction of the guide. Waveguides are layers with dimensions of the order of the acoustic wavelength, and as mentioned above, device structures in which thin film waveguides are used to guide acoustic waves are often called Love wave devices. In Love wave devices, the acoustic energy is genuinely confined to the surface of the device in a pure shear horizontal mode, leading to greater analytical sensitivity. Conventional waveguides include a wide range of materials including both inorganic and organic materials.
Although inorganic materials have been successfully used as waveguides in Love wave devices, organic polymeric materials are generally more advantageous because the rheology of such materials can be tailored for low acoustic losses, high stability of the waveguide under a liquid, and provide superior electrical insulation to the interdigitated electrodes (IDTs) of the device when used in a liquid. Furthermore, a wide variety of coating methods can be used to apply a polymeric waveguide in a device construction. Organic polymeric materials are also easy to (photo)image, so patterned coatings can be readily obtained.
Similarly, organic immobilization chemistries (which form a bridge between the waveguide and the biorecognition coating) are desirable because the rheology of such materials can be tailored for low acoustic losses, high stability of the immobilization chemistry layer under a liquid, especially when the thickness of this layer becomes appreciable when compared to the acoustic wavelength. Organic materials can also provide a superior adhesion bridge between the waveguide and the biorecognition coating. Furthermore, a wide variety of coating methods can be used to apply a polymeric immobilization layer in a device construction.
There is a continuing need for organic materials that can be used as the waveguide and/or immobilization chemistry in acoustic sensors.